It might just be that someone in your area has decided that breeding their Saint Bernard to their Lab is great and keeps doing it, so you're seeing a boom in the local population of this mix.

Around here there is someone who breeds their Australian Shepherd to their Rottie consistantly. I think there have been 3 litters at this point, and I have seen at least 6 of the resulting puppies just walking around town, and coming into the store where I work. They're pretty obvious a mix, so it's easy to distinguish them